Conventional sonars which present detected echoes in accordance with echo level information have such drawbacks as:
1. Information on relative echo intensities would be lost due to signal saturation phenomenon; PA1 2. Low-level signals would be masked, or hidden, by high-level signals and become impossible to distinguish; and PA1 3. Nothing could be distinguished from echo signals having the same intensity level.
Various improvements have so far been made to overcome these problems, including the use of a sonar receiver having a wide dynamic range, provision of an automatic gain control (AGC) and development of new signal processing techniques. Despite such efforts, if there is no level difference in received signals as stated in point 3 above, it is by no means possible to discriminate between them by using signal level information alone.
Should there exist a school of swimming fish, ultrasonic echoes returned from the fish school are shifted in frequency due to the so-called Doppler effect. If this shift in frequency (or in phase), known as the Doppler shift, can be detected without jeopardizing the sonar's ability to provide high-speed scanning (or real-time processing) and a wide searching area as well as its bearing and range resolutions, it would be possible to discriminate true targets among ultrasonic echoes from fixed targets such as the sea bottom. It would also be possible to recognize differences in moving velocities of fish schools, resulting in a significant improvement in target discriminating capabilities. For this reason, certain types of conventional sonars provide the ability to determine amounts of Doppler shifts (refer to Japanese Patent Publication No. 57-29975, for example).
Those conventional systems, however, have just been able to measure amounts of Doppler shifts of ultrasonic echoes arriving from only one particular direction and not provide high-speed measurement capabilities for Doppler shifts of ultrasonic echoes arriving from all directions. This is the reason why there has not been available yet a sonar capable of presenting moving velocities of underwater targets based on frequency information. It has long been desired therefore to develop a system that can provide real-time measurements of moving velocities of multiple fish schools throughout a wide searching area.